The continuing controversy regarding the precise role of respiration as an influence on craniofacial growth, development and morphology affects clinical specialties such as orthodontics, maxillo-facial surgery, otorhinolaryngology, allergy and pediatrics. The major reason for this controversy is the lack of unambiguous criteria for establishing what constitutes impaired nasorespiratory function or "mouth-breathing". Lacking unambiguous and objective criteria, it is difficult to confirm the presence, severity or etiology of the condition. This frequently may lead to unnecessary or inappropriate treatment. Since such treatments cannot be evaluated in terms of need or efficacy, the potential benefit of outcomes, either for orthodontic conditions or in terms of improving respiratory function, cannot be critically analyzed in terms of risk or cost. It is probable that many patients whose respiratory function falls within the range of normal physiologic variation for their age, sex and stature, receive treatments that produce no tangible benefits. Conversely, patients whose respiration may be significantly impaired, but whose craniofacial morphology appears normal or inconsistent with prevailing notions of the relationships between specific malocclusions and "mouth-breathing", may fail to be identified for treatment. We have made significant progress in the quantification of respiratory mode, percent nasal breathing and nasal airway conductance. Our experimental protocols can also now yield data on the probable location of the greatest constriction within the nasal passage, which may be anywhere between the anterior nares and the nasopharynx, where the adenoids reside. We have thus established diagnostic criteria for the presence, severity and possible cause of reduced nasal airflow. The goals for these studies include (1) establishing normative standards for nasorespiratory parameters, (2) evaluating and improving diagnostic criteria for impaired function, (3) quantifying the effects of currently prevalent orthodontic and surgical treatments for impaired nasal respiration, and (4) providing new basic information to promote a better understanding of the physiology of the upper respiratory tract. With the increase in demand for orthodontic treatment, these studies have considerable clinical significance to the fields of dentistry and those medical specialties concerned with craniofacial form/function and respiration.